"When is his birthday?"
The main function of the verb 是 is to serve as a copula of two nominals, i.e. tell us that "sth is sth". It cannot be used with adjectives as a copula, as adjectives in Chinese work like verbs, e.g. 大 "big" is literally "to be big" with that English "to be" already being expressed within it.
I'm kind of a beginner myself, but at least with adjectives you usually don't use it in positive sentences at all. With 不, you do use it with long adjectives but not with short ones. 我的字不好. vs. 他的西瓜不是新鮮. If you do use it, it emphasizes what you're saying, like "(but) you ARE beautiful!"